With recent advances in imaging technology, it is now feasible to use cross-sectional and Doppler echocardiography to assess the ductus arteriosus in neonates and older children. This study will seek to validate this technique and apply it to three groups of newborn infants to assess changes in morphology and hemodynamic effects of the ductus with time. Because echocardiography is noninvasive, serial studies can be performed without the risks of cardiac catheterization and new information can be obtained which was not availabale by other methods. Cross-sectional (2-D) echocardiography will be used to visualize the ductus arteriosus and measure the length and aortic and pulmonary orifice diameters. Doppler echocardiography will be used to examine the systolic and diastolic blood velocity patterns in the main pulmonary artery, ductus arteriosus and aorta to assess the hemodynamic effect of ductal patency. This study will attempt to clarify the morphologic variability of the ductus arteriosus including the changes during spontaneous closure, and will provide new information which could affect future attempts at pharmacologic manipulation of the ductus arteriosus.